Codel International Ltd

- Model 301 - Air Flow Monitor (AFM)



The TunnelTech 301 Air Flow Monitor (AFM) is CODEL’s industry proven tunnel air flow monitor. Tunnel ventilation systems are designed to enable a sufficient throughput of air to be maintained to ensure a safe operating environment for users. These systems consume large amounts of power and need to be operated as efficiently as possible. To do this it is necessary to know the effectiveness of the system in terms of air flow and direction developed within the tunnel. Three decades of development, knowledge and practical experience have been utilised to produce the advanced TunnelTech 301 air flow sensor that combines the reliability of ultrasonic technology and delivers superb accuracy and reliability at a very competitive price.


An ultrasonic transmitter and four receivers are contained in a single compact unit that can be mounted on the tunnel wall.

Sound waves from the transmitter Tx are received by the four receivers Rx1, 2, 3 and 4 as shown in figure 2. With no air flow the sound transmission time to the complementary pairs of receivers Rx1 and 4 and Rx2 and 3 will be equal. The sound wave received at Rx1 will be exactly in phase with that at Rx4. Similarly Rx2 will be in phase with Rx3. When air flow exists as shown in figure 2 the air flow opposes the sound transmission to receivers 1 and 2 along paths A and B thereby increasing the sound transmission time, while sound transmission to receivers 3 and 4 along C and D is enhanced thus reducing transmission time. The result is that a phase shift develops between Rx1 and Rx4, and between Rx2 and Rx3.

This phase shift is directly proportional to air velocity and direction. These phase shifts can be measured extremely accurately to provide a continuous and very reliable measurement of air velocity.

Two pairs of receivers are used in order to achieve a working air velocity range up to 20m/sec. The outer pair of sensors are used for low velocities and the inner pair for high velocities. The sensor contains no moving components and requires no routine maintenance.

Fully configurable analogue and alarm outputs are provided, an optional choice of either RS232 or RS485 outputs which can be utilised to deliver MODBUS protocol to a SCADA system located in the tunnel control centre.

Over the last 15 years CODEL tunnel sensors have been supplied to hundreds of road and rail tunnels throughout the world. Our impressive reference list includes Eurotunnel France, Mont Blanc Tunnel France, Dartford Tunnel UK, Lane Cove Tunnel Australia, Snow Mountain Tunnel Taiwan and the SMART Tunnel in Malaysia, plus many others throughout China, Italy, Germany, Switzerland and South Korea placing CODEL as a world leader in tunnel atmosphere monitoring.

CODEL’s tunnel sensor range is further extended by additional sensors for the measurement of CO, NO, Visibility and NO2.

  • Single point ultrasonic measurement - technology
  • Unaffected by traffic flow and sound reflections
  • No moving components providing maintenance free operation
  • Designed to withstand the harshest of tunnel environments
  • Measurement precision of +/- 0.2m/sec

  • Measurements: Air flow and direction
  • Measuring units: m/sec
  • Measurement Technique: Ultrasonic
  • Ambient Temperature: -20°C to +50°C
  • Power supply: 12V DC @1.5A
  • Construction: Corrosion resistant epoxy coated aluminium housing sealed to IP65
  • Analogue outputs: 2 x 4-20mA current outputs as standard, isolated, 500Ω maximum load, fully configurable
  • through TunnelTech software.
  • Relay Outputs: 2 x volt-free SPCO contacts, 50V, 1A maximum load, configurable as alarm contacts
  • Serial Data: For local connection to laptop PC using RS232 or optional RS485 interface unit

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